This invention relates to a hydrophilic treatment of films made of styrene resins and application of modifiers to improve, for example, the antistatic properties and slip properties of the films. More particularly, it relates to films to be mechanically processed with, for example, a bag-forming apparatus or a window film applicator, in particular, styrene resin films suitable for window film application.
Because of being stiff and highly transparent, styrene resin films have been frequently employed as food packaging films for lettuces, raw shiitake and bananas and window films (i.e., films to be applied to envelope windows).
Styrene resin films are employed as food packaging films, since they exhibit excellent food visibility due to the high transparency and high steam-permeability, thereby keeping the freshness of foods with much transpiration (for example, raw shiitake) over a prolonged period of time. Polyethylene films and polypropylene films, which have low steam-permeability, are unsuitable for packaging foods with much transpiration.
In particular, styrene resin films are frequently employed as envelope window films, since less stiff films (for example, polyethylene films, polypropylene films) suffer from troubles such as wrinkling in the step of windowing envelopes and therefore are unsuitable for this purpose.
As an example of the utilization of a styrene resin film in food packaging bags for keeping freshness, JP-A-8-230933 discloses a styrene resin film (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d). This film can be processed in sheet form into bags in which foods are filled by hand. However, this film has been subjected to no surface treatment, which brings about a problem of shear in bag-forming because of the insufficient slip properties and opening properties (i.e., easiness in opening the bags for filling foods therein) in the step of automatically filling foods (for example, shiitake) into the bags, in particular, processing the film into bags at a high speed and automatically filling foods in the bags.
In recent years, bananas are packaged in a bag formed by fuse-sealing three sides of a monolayer styrene resin film piece. To wrap a bulky food such as bananas, the bag should be provided with a wide opening. Thus, static electrification due to friction of the upper and lower film sheets makes it difficult to open the bag, thereby causing a problem that bananas can be hardly put into the bag.
To solve this problem, there have been reported styrene resin films coated with modifiers on the film surface. However, these styrene resin films suffer from various problems as will be discussed hereinafter.
As described above, it has been a practice to carry out various surface treatments for improving the slip properties of styrene resin film bases and imparting antistatic properties thereto to thereby give styrene resin films appropriately withstanding mechanical processing, for example, bag-formation and envelope window film application.
For example, JP-A-53-115781 proposes a method whereby a styrene resin film base surface is subjected to a hydrophilic treatment (for example, corona discharge) to give a surface tension of 400 to 550 xcexcN/cm and then a modifier containing an anticlouding agent and silicone oil is applied on the base surface. According to this technique, one surface of the film base is exclusively subjected to the corona discharge and the modifier is applied onto the thus treated surface. On the other hand, JP-A-10-119978 discloses a method of applying an antistatic agent onto both surfaces of a base material.
Recently, the processing speeds of bag-forming machines, in particular, window film applicators and automatic paper feeders have been elevated to 1,000 sheet/minute or more, over the machines currently in use (i.e., 400 to 600 sheet/minute). With this tendency toward higher processing speeds, it becomes necessary that a window film has two conflicting characteristics, namely, antistatic properties of both surfaces and adhesiveness to paper. It is true that the film disclosed by JP-A-10-119978 suffers from less trouble exclusively from the view point of static electricity. However, it is provided with no means of preventing film scratches. In addition, the problems of wrinkling and positioning error after adhesion are not completely solved in this case. Namely, the problem of static electricity can be solved merely by applying a large amount of an antistatic agent to the film base surfaces to thereby enhance the antistatic properties. However, this treatment brings about another trouble that the antistatic agent applied thickly prevents an adhesive from attaining the film base surfaces and solidifying thereon, thus causing wrinkling and positioning error.
On the other hand, JP-A-2-72050 discloses a styrene resin film containing a waxy antiblocking agent, while JP-A-2-72051 discloses a styrene resin film containing a granular antiblocking agent. Although the films reported in these documents show relieved film damage due to the improved slip properties, they still suffer from the problem of the frequent occurrence of troubles due to static electricity.
By the antistatic treatment on exclusively one surface according to JP-A-53-115781, the obtained film is hardly used for window film application. A styrene resin film roll set in a window film applicator is unwound and then brought into contact with metal rollers or rubber rollers before the adhesion of the film to the envelope paper. Since plural rollers are employed in the contact step, both surfaces of the film come into contact with the rollers and thus electrostatically charged. Unless the film has the antistatic properties on both surfaces, the film sheets, having been cut into a definite size, wind around each other immediately before coming into contact with the envelope paper, thus making continuous processing impossible.
Therefore, double-side application is employed in the thermoplastic resin film for envelope windowing according to JP-A-10-119978. However, this double-side application disclosed in this document suffers from the following problem.
Namely, the film for a window film applicator disclosed in this document is not subjected to any hydrophilic treatment on both surfaces but an antistatic agent is applied on both surfaces of the film in almost the same coating weights, followed by winding into a roll. The film roll thus formed is put into the window film applicator and then unwound before using as a film. In the unwinding step, however, there arises a problem that the antistatic agent, which has been applied onto both surfaces in the same weight, partly (or mostly in an extreme case) migrates from one surface to the opposite surface, depending on the winding tension and other environmental factors (for example, temperature, humidity) in the winding step and the storage conditions (for example, temperature, humidity) of the rolled film. As a result, the coating weight of the antistatic agent widely varies lengthwise. When the film surface having a large amount of the antistatic agent thereon is to be adhered to the envelope paper, the antistatic agent interferes the adhesion and thus causes positioning error between the film and the envelope window, thereby damaging the commercial value of the product.
The invention aims at imparting antistatic properties to both surfaces of a film to thereby provide a film which is suitable for mechanical processing with the use of, for example, a high-speed printer, a bag-forming machine or a high-speed window film applicator, in particular, a styrene resin film having improved adhesion properties to paper as required in an envelope window film applicator, and a process for producing the same.
The present inventors have conducted intensive studies to solve the above-described problems. As a result, they have successfully found out that the object can be achieved by imparting different surface tensions to the front and back surfaces of a film base through a hydrophilic treatment and then applying almost the same modifier compositions on both surfaces of the film base each in an appropriate coating weight. The invention has been completed based on this finding.
Accordingly, the invention provides a styrene resin film comprising a styrene resin film base subjected to a hydrophilic treatment on both surfaces and a composition containing at least one antistatic agent and an external slip agent applied onto each of the treated surfaces, wherein the ratio (xcex1/xcex2) of the surface tension (xcex1) of one surface (A) of the film base to the surface tension (xcex2) of the opposite surface (B) is from 1.15 to 1.72; the surface tension (xcex2) is from 350 xcexcN/cm to 450 xcexcN/cm; the surface tension (xcex1) is from 400 xcexcN/cm to 600 xcexcN/cm; and the coating weight of the composition on the surface (B) amounts to 25 to 95% by weight of the coating weight of the composition on the surface (A).
Moreover, the invention provides a process for producing a styrene resin film which comprises: performing a hydrophilic treatment so that the surface tension (xcex1) of one surface (A) of a film base is controlled to 400 to 600 xcexcN/cm, the surface tension (xcex2) of the opposite surface (B) of the film base is controlled to 350 to 450 xcexcN/cm and the surface tension ratio xcex1/xcex2 is controlled to 1.15 to 1.72; applying a composition containing at least an antistatic. agent and an external slip agent onto the surface (A); after drying, winding up the styrene resin film into a roll; and thus transferring the composition on the surface (A) to the surface (B).
Now, the invention will be described in detail.
The film base to be used in the invention contains as the main component a styrene resin. That is, the content of the styrene resin amounts to 50% by weight or more of the resin composition constituting the film. The styrene resin to be used in the invention is a transparent polymer containing 50% by weight or more of styrene monomer. Examples thereof include publicly known polymer resins such as polystyrene, styrene-acrylonitrile copolymer, styrene-methyl methacrylate copolymer and styrene-butadiene-methyl methacrylate terpolymer; so-called rubber-modified polystyrene resins containing synthetic rubber (for example, butadiene, styrene-butadiene random copolymer, styrene-butadiene block copolymer) dispersed therein as soft components; and block copolymers of styrene with conjugated dienes. Among all, it is favorable to use therefor general-purpose polystyrene (GPPS) or a resin prepared by dispersing a synthetic rubber in general-purpose polystyrene.
The styrene resin film to be used as the film base per se has a hydrophobic nature on both surfaces. When an aqueous modifier-containing solution prepared by dissolving an antistatic agent or an anticlouding agent in water is applied on the film surface, therefore, droplets of the aqueous modifier-containing solution are formed thereon and thus uniform application becomes impossible. In addition, an aqueous emulsion, which is employed as an adhesive in the step of adhering the film surface to envelope paper, should quickly attain the base surface and solidify thereon. Thus, it is needed to make the film surfaces hydrophilic. As will be described hereinafter, the hydrophilic treatment can be performed by using an acid (for example, sulfuric acid, nitric acid) or by a plasma treatment (for example, corona discharge).
It is essentially required in the invention that the styrene resin film base surfaces are subjected to the hydrophilic treatment so as to give a surface tension ratio of the front and back surfaces (xcex1/xcex2) falling within a specific range. To obtain a styrene resin film suitable for a window film applicator or an automatic paper feeder (hereinafter referred to simply as a window film applicator in some cases), it is particularly necessary to vary the extent of the hydrophilic treatment on these surfaces. In case of using the styrene resin film as a window film of an envelope or a sheet, the styrene resin film is to be adhered to the inside of the adherend (for example, an envelope) and thus the film surface (Axe2x80x2) serves as the innermost surface of the envelope. When the film surface (Axe2x80x2) is electrostatically charged in the step of window film application, the film surface (Axe2x80x2) attracts the adherend owing to the static electricity and thus the envelope can be hardly opened, thereby making it impossible to put papers into the envelope. Since the papers are enveloped or located while being in contact with the inner surface of the adherend, the film surface (Axe2x80x2) is electrostatically charged due to the frictional charge and, in its turn, the papers per se are also electrostatically charged. In an automatic paper feeder of strict type, the film surface (Axe2x80x2) is electrically charged and papers are also charged and thus the papers cannot be inserted due to the static electricity. Accordingly, it is necessary that the film surface (Axe2x80x2) has sufficiently higher antistatic properties than the film surface (Bxe2x80x2).
On the other hand, the film surface (Bxe2x80x2) should have antistatic properties too so as to prevent the problem that film sheets wind around each other due to the static electricity in the step of running the films on plural rollers in a window film applicator. It is also necessary that the film surface (Bxe2x80x2) has appropriate adhesion properties to the adherend. To achieve these objects, the film surface (Bxe2x80x2) should have lower antistatic properties by a specific ratio than the film surface (Axe2x80x2) and ensure the adhesion of the adhesive. It is therefore needed in the invention to control the ratio of the surface tension (xcex1) of the surface (A) of the film base to the surface tension (xcex2) of the other surface (B) of the film base (i.e., xcex1/xcex2) to 1.15 to 1.72. Particular reason therefor is as follows.
When the surface tension ratio falls within the range of from 1.15 to 1.72, a large amount of the modifier composition remains on the film surface (Axe2x80x2) after unwinding the film roll, even though the film is affected by the winding tension in the winding step or the storage temperature and humidity. Thus, both surfaces can sustain respectively the appropriate antistatic properties and attainment times of the adhesive. It is preferable that the surface tension ratio is from 1.20 to 1.60.
When the surface tension ratio is less than 1.15, it becomes impossible to control the coating weights of the modifier composition on the film surfaces (Axe2x80x2) and (Bxe2x80x2), depending on the winding tension in the winding step or the storage temperature and humidity, as in the case where the base surfaces are not subjected to the hydrophilic treatment. When the surface tension ratio exceeds 1.72, on the other hand, the peeling force between film sheets becomes excessively high, in spite of the modifier composition applied thereon, and thus the film roll can be hardly unwound.
As described in Examples 3 and 5 of JP-A-10-119978, it is obvious that the coating weights of the modifier composition on the film surfaces (Axe2x80x2) and (Bxe2x80x2) cannot be anticipated, in case of applying the composition on these surfaces without performing the hydrophilic treatment. Thus, it is advantageous as a food packaging film too that the film surfaces (Axe2x80x2) and (Bxe2x80x2) have different properties from each other as described above.
When a styrene resin film is laminated on another resin film as in JP-A-8-230933, the surface (Bxe2x80x2) may be employed as the film surface on which the other film is laminated. Thus, the adhesion strength can be maintained on the surface with less coating weight while the film surface (Axe2x80x2) superior in the slip properties and the antistatic properties can regulate the occurrence of shear in bag-forming during the automatic bag-forming process.
In a bag having a large opening for wrapping, for example, bananas, the film surface (Axe2x80x2) is made inside so that the bag can be easily opened owing to the antistatic properties of the surface (Axe2x80x2).
Similarly, particular surface tension values of the base surfaces are restricted to certain ranges for the following reasons, thereby facilitating the achievement of the objects of the invention.
The surface tension (xcex1) of the film base surface (A) is controlled to a range of from 400 xcexcN/cm to 600 xcexcN/cm by the hydrophilic treatment. So long as the surface tension falls within this range, the modifier composition can be more uniformly applied on the base surface and the film base surface can be adequately activated so that blocking of film sheets (i.e., a phenomenon causing an increase in the peeling force between film sheets) scarcely arises. The surface tension (xcex1) preferably ranges from 430 xcexcN/cm to 580 xcexcN/cm, still preferably from 450 xcexcN/cm to 550 xcexcN/cm.
The surface tension (xcex2) of the opposite surface (B) of the film base is from 350 xcexcN/cm to 450 xcexcN/cm. When this surface tension is 350 xcexcN/cm or more, the film surface shows an improved affinity for the adhesive and thus the attainment of the adhesive to the base surface and the solidification thereon can be quickly completed. As a result, the adhesion force between the film and paper can be enhanced and the adhesion time can be shortened, which is appropriate particularly in high-speed window film application at 1,000 sheet/minute or more. It is preferable that the surface tension (xcex2) is 380 xcexcN/cm or more.
The upper limit of the surface tension (xcex2) is 450 xcexcN/cm. When the surface tension of the film base surface (B) exceeds 450 xcexcN/cm, the surface tension of the film base surface (A) becomes at least 515 xcexcN/cm because of the definition of the surface tension ratio. In such a case, the film roll can be hardly unwound and the obtained film is hardly used especially in high-speed window film application at 1,000 sheet/minute or more. This is because the coating weight on the film surface (Bxe2x80x2) is smaller than the coating weight on the film surface (Axe2x80x2) and therefore the peeling force between film sheets is excessively elevated due to the synergistic effect with the activated film base surface (A), when the surface activity of the film surface (B) is too much elevated by the hydrophilic treatment. It is preferable that the surface tension (xcex2) of the film base surface (B) is 430 xcexcN/cm or less. As described above, the peeling force between film sheets is elevated by the synergistic effect of the film surface (A), which has been activated by the hydrophilic treatment, and the film base surface (B) and, in its turn, the peeling force between the film and the adherend is seemingly elevated too.
Next, the composition containing an antistatic agent and an external slip agent (hereinafter referred to as the modifier composition) which is to be applied onto the film base surfaces (A) and (B) will be illustrated in greater detail.
Examples of the antistatic agent include conductive fillers such as carbon black and nickel powder and surfactants having antistatic properties.
The former antistatic agents (i.e., conductive fillers) leak static electricity due to the surface contact among conductive particles, while the latter antistatic agents (i.e., antistatic surfactants) leak static electricity because of the hygroscopic or ionic natures thereof. It is not favorable to use such a conductive filler, since it should be applied in a large amount to ensure the leakage of the static electricity due to the surface contact among the microparticles and thus the transparency is worsened thereby. Therefore, it is preferred to use the latter ones (i.e., surfactants).
The external slip agent is used in order to improve the slip properties. When present on the surface, it imparts lubricating action or slip properties on the basis of the principle of the mechanism of rollers.
Examples of the external slip agent imparting lubricating action include silicone oils, waxes and surfactants as will be described hereinafter. As an example of the silicone oils, dimethyl silicone oil may be cited. Examples of the waxes include amide type lubricants (for example, stearic acid amide, erucic acid amide) and ester type lubricants (for example, butyl stearate, stearic acid monoglyceride). Examples of microparticles include silicone dioxide, talc and calcium carbonate. Although oil-soluble lubricants (for example, silicone oils and waxes) are appropriate for improving the slip properties, it is unfavorable to use these lubricants since they lower the sealing strength in bag-forming or worsen the adhesion properties of the film to paper.
Preferable examples of the antistatic agent include surfactants having antistatic properties as will be described hereinafter, polyoxyethylenealkylamines and polyoxyethylene polyoxypropylene glycol ether. Preferable examples of the external slip agent include inorganic particles as will be described hereinafter and polyether-modified silicones.
The external slip agent is added at a weight ratio of generally from 0.01 to 3, preferably from 0.05 to 2.5, based on the antistatic agent.
As preferred constitution of the invention, the following three types of modifier compositions may be presented. Namely, the first modifier composition contains a surfactant as the antistatic agent, inorganic microparticles as the external slip agent, and further a water soluble polymer; the second modifier composition contains a surfactant as the antistatic agent, a specific polyether-modified silicone as the external slip agent, and further a water soluble polymer; and the third modifier composition contains a specific polyoxyethylenealkylamine or polyoxyethylene polyoxypropylene glycol ether as the antistatic agent and a specific polyether-modified silicone as the external slip agent.
In some cases, either the antistatic agent or the external slip agent has the function of the other too.
Now, the first modifier composition in the invention will be illustrated.
In the first modifier composition, use can be made of a surfactant having antistatic properties as the antistatic agent. Examples of the surfactant for use herein include aninonic surfactants (for example, carboxylic acid salts, sulfonic acid salts, sulfate salts, phosphate salts, phosphonic acid salts), cationic surfactants (for example, amine salts, quaternary ammonium salts, sulfonium salts), amphoteric surfactants (for example, betaine type surfactants, imidazoline type surfactants), and nonionic surfactants (for example, polyhydric alcohol type fatty acid monoglycerol esters, fatty acid polyglycol esters, fatty acid sorbitan esters, fatty acid sucrose esters, fatty acid alkanolamide-polyethylene glycol fused fatty acid, aliphatic alcohols, aliphatic amines, alkyl phenols, polypropylene glycol). It is preferable to use an amphoteric surfactant or a nonionic surfactant, since stable antistatic properties can be obtained thereby. It is also possible to use a combination of two or more of these surfactants.
Examples of the inorganic microparticles include microparticles of silicon dioxide, silicates, synthetic zeolite, calcium carbonate and magnesium carbonate. Owing to the function of these inorganic microparticles, the slip properties of the film can be improved and the peeling force between film sheets can be lowered after winding into a roll. These inorganic microparticles should have an inner surface area preferably ranging from 0.5 to 4.0 m2/g in terms of specific surface area. The preferable particle diameter ranges from 1 to 7 xcexcm, still preferably from 2 to 5 xcexcm, in terms of number-average particle diameter. It is favorable to use silicon dioxide microparticles in view of the particle diameter and the inner surface area. Because of having the inner surface area, portions of the surfactant and the water soluble polymer are incorporated into the microparticles and thus the fixing force of the surfactant can be elevated, compared with the case of using the water soluble polymer alone. When the number-average particle diameter falls within the range of 1 to 7 xcexcm, the microparticles scarcely fall out during being in contact with rollers of a window film applicator and the fixing power of the surfactant is elevated.
Examples of the water soluble polymer include polyvinyl alcohol, polyvinyl pyrrolidone, sodium polyacrylate, sodium polystyrenesulfonate and methylcellulose. Among all, polyvinyl alcohol is preferable from the viewpoints of solubility in water and fixing power.
It is further preferable that polyvinyl alcohol has a degree of saponification of from 40 to 99% by mol, still preferably from 60 to 95% by mol. When the degree of saponification falls within this range, an aqueous solution of the polyvinyl alcohol can be easily prepared and the film strength is not deteriorated. As a result, there arise no fear of staining rollers, when the film comes into contact with the rollers. The molecular weight of the water soluble polymer generally ranges from 100 to 15,000, preferably form 200 to 5,000. Use of the water soluble polymer facilitates the uniform application of the modifier composition, in addition to the effects as will be described hereinafter.
To satisfy the requirements in case of processing the film according to the invention with the use of a high-speed printer, a bag-forming machine or a high-speed window film applicator, the first modifier composition contains not only the surfactant as described above but also the water soluble polymer and the inorganic microparticles. By adding the water soluble polymer and the inorganic microparticles, the surfactant is strongly fixed to the film surface and thus rollers of various apparatuses can be prevented from staining. Moreover, the inorganic microparticles can be prevented from falling out from the film surface due to the synergistic effect of the water soluble polymer and the inorganic microparticles.
In the first modifier composition, the most suitable mixing ratio by weight of (a) the surfactant, (b) the water soluble polymer, and (c) the inorganic microparticles (i.e., (a):(b):(c)) is 1:0.03 to 3:0.01 to 1.25.
When the composition ratio of the water soluble polymer is from 0.03 to 3, it exerts a strong fixing power to the surfactant and thus the surfactant scarcely peels off from the film surface. In this case, the effects of the surfactant can be easily exerted, and thus the antistatic properties are improved.
When the composition ratio of the inorganic microparticles is from 0.01 to 1.25, the fixing effect of the surfactant can be achieved owing to the synergistic effect of the water soluble polymer and the inorganic microparticles and thus the inorganic microparticles scarcely fall out from the film surface.
The desired level of the antistatic properties or the slip properties (i.e., the coefficient of dynamic friction) varies depending on the purpose of use.
In case of using the film as a food packaging film and processing into bags while automatically filling a food therein, for example, automatic filling can be carried out without causing shear in bag-forming by controlling the coefficient of high-speed dynamic friction between the film and a metal to 0.15 to 0.57 and controlling the half-life of electrostatic attenuation (20xc2x0 C., relative humidity 25%) to 300 seconds or less.
In case of using the film of the invention in window film application, for example, high-speed processing (1,000 sheet/min or more) can be carried out without causing any troubles such as wrinkling by controlling the coefficient of high-speed dynamic friction between the film and a metal to 0.15 to 0.35 and controlling the half-life of electrostatic attenuation (20xc2x0 C., relative humidity 25%) to 90 seconds or less.
To satisfy these requirement, the modifiers are to be applied on the film base surfaces respectively in the following weights: (a) from 2.0 to 15 mg/m2 (preferably from 3 to 12 mg/m2), (b) from 0.5 to 6 mg/m2 (preferably from 0.5 to 4 mg/m2), and (c) from 0.2 to 2.5 mg/m2 (preferably from 0.3 to 2.0 mg/m2, still preferably from 0.3 to 1.8 mg/m2). A coating weight of the surfactant of 2 to 15 mg/m2 is suffice for achieving antistatic effect. In this case, moreover, the inorganic microparticles are not embedded in the coating layer but appropriately coated with the surfactant. When processed with a window film applicator, therefore, the thus obtained film suffers from little wrinkling or positioning error caused by static electricity.
The water soluble polymer is employed together with the surfactant in order to retain the inorganic microparticles on the film surface. Since the surfactant alone can achieve only an insufficient effect of retaining the inorganic microparticles, the water soluble polymer is added as a so-called adhesion enhancer for strengthening the adhesive force. To establish the aimed effect, the coating weight of the water soluble polymer preferably ranges from 0.5 to 6 mg/m2. When the coating weight of the water soluble polymer falls within this range, a sufficient effect of retaining the inorganic microparticles can be established and thus scratches are scarcely formed due to the fall-out of the inorganic microparticles from the film surface. In this case, moreover, the antistatic properties of the surfactant are not deteriorated and the slip properties are not deteriorated due to the excessively elevated adhesiveness on the film surface.
When the coating weight of the inorganic microparticles is from 0.2 to 2.5 mg/m2, a sufficient number of inorganic microparticles can be dispersed to form projections thereby achieving the effect of improving the slip properties. In addition, little inorganic microparticles fall out from the film and, therefore, the film suffers from few scratches in this case.
Next, the second modifier composition will be illustrated.
The second modifier composition contains a polyether-modified silicone, a surfactant and a water soluble polymer. One of the characteristics of this combination resides in that the slip properties can be improved thereby without resort to any inorganic microparticles and thus it is unnecessary to give attention to prevent the fall-out of inorganic microparticles from the film. The inventors have found out a modifier capable of achieving slip properties without resort to inorganic microparticles, namely, a polyether-modified silicone and thus disclosed the same as the second modifier composition.
The polyether-modified silicone disclosed in the invention has a structure represented by the following formula (1) wherein the methyl groups of dimethyl silicone have been partly modified with polyoxyethylene and polyoxypropylene groups. 
In formula (1), R1 represents a hydrogen atom or a lower alkyl group having 1 to 4 carbon atoms; m is an integer of from 0 to 80 and n is an integer of from 1 to 30, provided that m+n is an integer of from 1 to 100 and the ratio n/(n+m) is from 0.1 to 1.0; and a is an integer of from 5 to 30 and b is an integer of from 0 to 30, provided that a+b is an integer of from 5 to 60 and the ratio a:b is from 100:0 to 100:50.
In the polyether-modified silicone of formula (1), it is still preferable that m is an integer of from 1 to 30; the ratio n/(n+m) is from 0.3 to 1.0; a+b is from 5 to 15; and the ratio a:b is from 100:0.
In case where the polyether-modified silicone alone is used as a modifier to achieve satisfactory antistatic properties and slip properties, the polyether-modified silicone should be applied onto the film in a larger weight than, for example, the surfactant. Due to the low surface tension of silicone molecules, there is observed a tendency that the water-repellency of the film surface is unnecessarily elevated. Therefore, the film sometimes shows poor adhesion properties to an adhesive and thereby causes positioning error, when it is to be adhered to paper by using a window film applicator.
In styrene resin films, it is frequently observed that films carrying modifiers (for example, surfactants) applied thereon are recycled and reused. Since the polyether-modified silicone is incompatible with styrene resins, films containing the polyether-modified silicone in a large amount are whitened and worsened in transparency, which makes the recycle of these films difficult.
In the second modifier composition, the polyether-modified silicone is used together with a surfactant and, therefore, the content of the polyether-modified silicone can be reduced. The surfactant to be used in this case may be an arbitrary one selected from among the anionic, cationic and nonionic surfactants as described above. In this second combination, the water soluble polymer is further added so that the water soluble polymer fixed to the film base surface and retain the surfactant. Owing to this constitution, the surfactant can be uniformly dispersed on the film base surface, thereby achieving stable antistatic properties.
In the second modifier composition, the preferable composition ratio by weight of (a) the surfactant, (b) the water soluble polymer, and (c) the polyether-modified silicone (i.e., (a):(b):(c)) is 1:0.03 to 2.0:0.1 to 5.7. When the composition ratio falls within this range, the obtained film is appropriately processed with the use of, for example, a window film applicator.
It is favorable that the composition contains from 10 to 85% by weight of the polyether-modified silicone, from 15 to 90% by weight of the surfactant and from 3 to 30% by weight of the water soluble polymer.
By controlling the content of the polyether-modified silicone to 10% by weight or more, appropriate slip properties, sufficient slip properties and an antiblocking effect (a decrease in peeling force) can be obtained. As a result, the film scarcely suffers from positioning error or winding in the step of, for example, window film application. Moreover, the film shows an adequate adhesive force to paper and is free from any decrease in transparency in the course of recycling.
By controlling the content of the surfactant to 15% by weight or more, good antistatic properties can be imparted to the film, in particular, under less humid conditions (20xc2x0 C., relative humidity 20%). As a result, the film scarcely suffers form blocking or positioning error in the step of envelope window film application. In addition, this film is excellent in the friction between film sheets and antiblocking properties and, therefore, scarcely winds around each other in envelope window film application. Moreover, it shows an appropriate peeling force between film sheets, which ensures stable unwinding of the film roll.
By further controlling the content of the water soluble polymer to 3 to 30% by weight, the surfactant can be uniformly dispersed even though the polyether-modified silicone has a low surface tension. Thus, the surfactant can be uniformly fixed to the film base surface. This is seemingly established by the affinity of the water soluble polymer for the surfactant. The obtained film is less sticky and the surfactant is uniformly dispersed therein, thereby showing little scatter in the slip properties and antistatic properties.
From the viewpoint of the balance of the properties as described above, it is still preferable that the contents of the polyether-modified silicone, the surfactant and the water soluble polymer are controlled respectively to 40 to 60% by weight, 60 to 40% by weight and 5 to 25% by weight.
The coating weight of the modifier composition onto the film base surface preferably ranges from 2 to 30 mg/m2, still preferably from 2 to 15 mg/m2. When the coating weight falls within this range, sufficient antistatic properties and slip properties can be established and the film suffers from little positioning error when used in, for example, window film application. In recycling, moreover, the transparency and antiblocking properties of the film are not affected thereby.
Now, the third modifier composition will be illustrated. The third modifier composition contains a polyether-modified silicone represented by formula (1) as described above, a polyoxyethylenealkylamine represented by the following formula (2), or a polyoxyethylene polyoxypropylene glycol represented by the following formula (3). 
In formula (2), R2 represents an alkyl group having 8 to 22 carbon atoms; x is an integer of from 1 to 30; and y is an integer of from 1 to 30. 
The compound represented by formula (3) has a molecular weight of from 1,500 to 15,000 and the total weight of the repeating unit d and the repeating unit f amounts to 40 to 95% of the whole molecule.
Different from the first and second modifier compositions, the third modifier composition is free from any inorganic microparticles and thus it is unnecessary to give attention to prevent the fall-out of inorganic microparticles from the film. Furthermore, the third modifier composition is advantageous in that it can be produced economically because of being free from any water soluble polymer.
As described above, there are various types of surfactants. In case where a polyether-modified silicone is employed without adding any water soluble polymer, use is made of the surfactant represented by formula (2). When a polyether-modified silicone is used together with a fatty acid amide as disclosed in JP-A-10-119978, the slip properties and antiblocking effect characteristic to the modified silicone are inhibited. In this case, moreover, the antistatic properties characteristic to the fatty acid amide are also worsened and thus the object of the invention cannot be achieved.
In the third modifier composition, it is unnecessary to further add any water soluble polymer in order to improve the application properties. This is because the polyether-modified silicone is highly compatible with the polyoxyethylenealkylamine and thus the modifier composition can be uniformly applied onto the film base surface without adding any water soluble polymer.
The polyoxyethylenealkylamine of formula (2) disclosed in the invention can be obtained by an addition reaction between an aliphatic amine having 8 to 22 carbon atoms and ethylene oxide, or a dehydration reaction between an aliphatic amine and polyoxyethylene. Among all, it is favorable to use an aliphatic amine having 8 to 18 carbon atoms in view of the antistatic properties and slip properties of the film and the affinity with the polyether-modified silicone. For example, use may be made of saturated aliphatic amines such as laurylamine, myristylamine or autearylamine either alone or as a mixture with unsaturated aliphatic amine(s) such as oleilamine (for example, higher aliphatic amines obtained form coconut oil or beef tallow). It is still preferable to use a polyoxyethylenealkylamine of formula (2) wherein x is an integer of from 1 to 15 and y is an integer of from 1 to 15.
The polyoxyetylene polyoxypropylene glycol of formula (3) disclosed in the invention is a high-molecular weight nonionic surfactant having polyoxyethylene as a hydrophilic group and polyoxypropylene as a hydrophobic group. The hydrophilic/hydrophobic balance can be controlled depending on the values a, b and c in the formula.
In the invention, the content of the repeating units (d+f) amounts to 40 to 95% by weight, preferably 50 to 85% by weight, of the whole molecule from the viewpoints of the antistatic properties, the compatibility with the polyether-modified silicone and the slip properties.
The most suitable composition ratio (a) the polyether-modified silicone to (b) the polyoxyethylenealkylamine or polyoxyethylene polyoxypropylene glycol (i.e., (a):(b)) is 1:0.1 to 9.0. In case where the film is to be used in mechanical processing such as window film application, it is favorable to use the composition ratio as defined below.
Namely, it is favorable that the third modifier composition contains from 85 to 100% by weight of the polyether-modified silicone and form 15 to 90% by weight of the polyoxyethylenealkylamine or polyoxyethylene polyoxypropylene glycol. By controlling the composition ratio to this level, appropriate slip properties, sufficient slip properties and an antiblocking effect (a decrease in peeling force) can be obtained. As a result, the film scarcely suffers from positioning error or winding in the step of, for example, window film application. Moreover, the adhesion of the film to an adherend is not inhibited and the film is free from any decrease in transparency in the course of recycling.
By controlling the content of the polyoxyethylenealkylamine or polyoxyethylene polyoxypropylene glycol to 15% by weight or more, good antistatic properties can be imparted to the film, in particular, under less humid conditions (20xc2x0 C., relative humidity 20%). As a result, the film scarcely suffers from blocking or positioning error in the step of envelope window film application. By controlling the content thereof to 90% by weight or less, improvement can be made in the friction between film sheets and antiblocking properties and, therefore, the film scarcely winds around each other in envelope window film application. Moreover, it shows an appropriate peeling force between film sheets, which ensures stable unwinding of the film roll.
In an application system wherein either the polyether-modified silicone, the polyoxyethylenealkylamine, or the polyoxyethylene polyoxypropylene glycol is employed alone, it is difficult to satisfy all of the requirements for the antistatic properties, slip properties, antiblocking effect and transparency. By using these components together, the uniform dispersion of the polyether-modified silicone on the film surface can be facilitated and the slip properties and the antistatic properties can be synergistically improved. At the same time, the antiblocking effect can be established.
From the viewpoint of the balance of the properties as described above, it is still preferable that the contents of the polyether-modified silicone and the content of the polyoxyethylenealkylamine or the polyoxyethylene polyoxypropylene glycol are controlled respectively to 40 to 60% by weight and 60 to 40% by weight.
The modifier composition is applied to each surface of the film base in a weight of from 2 to 30 mg/m2, preferably from 2 to 15 mg/m2. When the application dose falls within this range, the antistatic properties are controlled to the adequate level and thus positioning error scarcely arises in bag-forming or window film application. In this case, moreover, the appropriate adhesion properties of the film to paper are ensured and the transparency and antiblocking effect of the film are not adversely affected in the course of recycling.
Next, the total coating weight of the modifier composition according to the invention will be described in detail.
The composition ratio of each modifier disclosed in the invention and the coating weight thereof on the film base surface have been described above. In case where the film according to the invention is to be used in mechanical processing, in particular, envelope window film application, it is recommended that the total coating weights of the modifier compositions onto the film surfaces (Axe2x80x2) and (Bxe2x80x2) are controlled respectively to definite ranges, as will be described hereinbelow.
Although the antistatic properties and anti-clouding properties are improved with an increase in the coating weight of the modifier composition, it is preferable for the following reason that the coating weights are restricted in a film for envelope window. More particularly speaking, it is favorable that the coating weight of the modifier composition onto the film surface (Axe2x80x2) ranges from 4.5 mg/m2 to 30 mg/m2 while that the coating weight of the modifier composition onto the film surface (Bxe2x80x2) ranges from 2.0 mg/m2 to 15 mg/m2.
When the coating weight onto the film surface (Axe2x80x2) falls within the range as defined above, sufficient antistatic properties can be established and yet the film shows no stickiness. When the coating weight onto the film surface (Bxe2x80x2) falls within the range as defined above, sufficient antistatic properties can be established and the adhesion time of the film to paper can be shortened. This is because the problem of the prevention by the modifier composition of the attainment of an adhesive to the film base surfaces can be overcome as described above.
The film according to the invention, which has the constitution as described above, has the following physical properties in case of using any modifier composition, so long as the requirements for the composition ratio and total coating weight as defined above have been satisfied.
Regarding the electrostatic properties on each film surface, the film surface (Axe2x80x2) has a half-life (JIS L 1094: measured at 20xc2x0 C., relative humidity 20%) of 90 seconds or less (still preferably 60 seconds or less), while the film surface (Bxe2x80x2) has a half-life of 300 seconds or less. When the half-life of the film surface (Axe2x80x2) is controlled to this level, papers can be surely enclosed in envelopes and film sheets do not wind around each other in, for example, an automatic paper feeder.
Next, the roughness of the film base surfaces (A) and (B) will be described in detail.
It is recommended in the invention that the center-line average of surface roughness parameter (hereinafter referred to simply as xe2x80x9csurface roughness parameterxe2x80x9d) is regulated within a specific range. The surface roughness parameter is measured by using a surface shape analyzer (SAS-2010 manufactured by Meishin Koki K.K.), extracting the roughness curves on definite lines on the film base surface before the application of the modifier, dividing the area of a part surrounded by a roughness curve of 1 mm in length and the center line with the measurement length (i.e., 1 mm) to thereby determine the average deviation in each of the longitudinal and transverse directions, and then calculating the longitudinal and transverse values.
In a preferred embodiment of the invention, the surface roughness parameter of the film base surface preferably ranges from 0.4 to 2.2 xcexcm, still preferably from 0.42 to 2.0 xcexcm.
In the invention, the surface characteristics of the film are improved and slip properties are imparted to the film by applying a modifier composition containing inorganic microparticles having a particle diameter falling within a specific range (i.e., the first modifier composition) to give a coating weight falling within a specific range, or by applying a polyether-modified silicone to give a coating weight falling within the specific range as described above. It has been found out that relatively gentle convexo-concave on the film base surface per se contributes to the effective performance of a high-speed mechanical processing operation.
That is to say, when the surface roughness parameter is 0.4 to 2.2 xcexcm, the film surface becomes smooth and thus suffers from little frictional scratches during high-speed operation on metal rollers. In this case, favorable slip properties are established too. As a result, the film scarcely undergoes wrinkling or positioning error even on, for example, a vacuum drum sucking the film at a high speed in envelope window film application.
In general, slip properties are evaluated on the basis of coefficient of dynamic friction. However, the measurement at less than 10 m/minute, which is commonly employed in the art, is insufficient in evaluating the slip properties during a high-speed operation in mechanical processing with the use of, for example, a bag-forming machine or a high-speed window film applicator. In the invention, therefore, a coefficient of high-speed dynamic friction ranging from 0.1 to 0.35 (between the film and a metal (specular stainless face) determined at a high speed of 30 m/min) is employed as the reference. It is obvious in the coefficient of high-speed dynamic friction that the coefficient of dynamic friction is elevated when the surface roughness parameter exceeds 2.2 xcexcm. When the film according to the invention is used in envelope window film application, it is preferable that the coefficient of high-speed dynamic friction of the film falls within the range as defined above. It is still preferable that both surfaces of the film have the coefficients of high-speed dynamic friction falling within this range. When the film is to be used for other purposes, however, this factor is not essentially required.
In the bag-forming machine of the automatic feeding type as described above, for example, a film having a coefficient of high-speed dynamic friction of 0.15 to 0.57 can be used without any troubles.
The surface roughness parameter can be controlled within the range as defined above by appropriately adding a rubber-modified polystyrene resin, which is a styrene resin, to the resin composition. In this case, it is preferable to add about 3 to 24% by weight of the rubber-modified polystyrene resin, though the content varies depending on the diameter of rubber particles contained in the rubber-modified polystyrene resin. Alternatively, the surface roughness parameter can be controlled by adding a component dispersible as particles in the rubber-modified polystyrene resin, for example, aromatic vinyl hydrocarbon/conjugated diene block copolymers or organic or inorganic microparticles other than the styrene resin.
In case of using microparticles, it is preferable to add from about 1% by weight to 10% by weight of the microparticles, though the content varies depending on the diameter and constituting resin of the microparticles.
The microparticle diameter preferably ranges from 0.5 to 10 xcexcm. When the content of the microparticles falls within the range defined above, the microparticles can be uniformly dispersed all over the film surface to give gentle convexo-concave, thereby achieving a coefficient of dynamic friction at the desired level. When the diameter of the microparticles falls within the range defined above, appropriate convexo-concave can be obtained.
By biaxially stretching such a film composition as described above, the granular components in the film affect the film surface shape and thus contribute to the formation of relatively gentle convexo-concave.
Next, the adhesion time will be illustrated in detail by citing a case of applying the film to an envelope window by way of example. (The method for measuring the adhesion time employed herein will be described in detail hereinafter.) In a window film applicator, a film is adhered to an envelope paper almost simultaneously with the application of an adhesive to the film. The adhesion is carried out by putting the film on a metal cylinder under high-speed rotation, feeding the envelope paper along the tangent line of the cylinder, and thus bringing the film into contact with the envelope paper. At the point of the contact of the film with the envelope paper, adhesion is performed under a shear force in the tangent direction of the cylinder. Accordingly, it is highly important in high-speed window film application to quicken the attainment of the adhesive to the base surface and the solidification thereon, thereby preventing positioning error between the film and the window frame.
Although an aqueous emulsion type adhesive can be quickly absorbed by the envelope paper and fixed thereon, it takes a somewhat long time (i.e., the adhesion time) that the adhesive attains the film base surface because of the antistatic agent present on the film surface. It is preferable that the adhesion time of the film base surface to the paper is not longer than 30 seconds. When the adhesion time falls within this range, the adhesive can sufficiently quickly attain the film base surface and thus positioning error scarcely arises between the film and the window frame and the commercial value of the envelope is not worsened. It is still preferable that the adhesion time is not longer than 25 seconds, still preferably not longer than 20 seconds. It is also preferable that the difference between the adhesion time of the film surface (Axe2x80x2) carrying the composition applied thereon to the paper and the adhesion time of the opposite film surface (Bxe2x80x2) to the paper is 1.0 second or longer and the adhesion time of the film surface (Bxe2x80x2) to the paper is not longer than 30 seconds.
Now, the production process according to the invention will be described in detail.
A styrene resin, which optionally contains publicly known additives (for example, a heat stabilizer, an antioxidant, a plasticizer), is molten and kneaded in an extruder and then stretched by the tentering method or the inflation method to give a film of a definite thickness.
In case of using the tentering method, either simultaneous biaxial stretching or successive biaxial stretching may be selected. It is desirable that the stretching is carried out at a temperature higher, by from 20xc2x0 C. to 40xc2x0 C., than the Vicat softening point of the styrene resin.
When stretched at a temperature lower than the lower limit as defined above (i.e., the Vicat softening point+20xc2x0 C.), the obtained film has a high rigidity. As a result, the film sheet can be hardly fed into a guide roll in a bag-forming machine of automatic filling type, which causes sealing failure. In the step of window film application, the film cannot follow up the envelop paper and thus causes positioning error. When the stretching temperature exceeds the upper level (i.e., the Vicat softening point+40xc2x0 C.), the obtained film becomes less stiff. As a result, the film cannot follow up the guide roll in a bag-forming machine of automatic filling type, which causes sealing failure. In this case, the film suffers from wrinkling in the course of the window film application.
In the stretching temperature range as defined above, the percent of stretch is adjusted to 2 to 17 both in the longitudinal and transverse directions. In order to enhance the film strength by imparting orientation properties and achieve uniform stretching, it is still preferable that the percent of stretch is adjusted to from 4 to 12. It is also desirable that the stretch ratio (longitudinal percent of stretch/transverse percent of stretch) falls within a range of from 1 to 1.3.
In the inflation method, the film is stretched 2- to 17-fold while controlling the temperature in the bubble chamber so that the stretching is started at a temperature higher, by from 30xc2x0 C. to 90xc2x0 C., than the Vicat softening point, while the bubble center temperature is adjusted to a point higher, by from 20xc2x0 C. to 60xc2x0 C., than the Vicat softening point. In order to enhance the film strength by imparting orientation properties and achieve uniform stretching, it is still preferable that the percent of stretch is adjusted to from 4 to 12.
The thickness of the film having been stretched according to the invention is not particularly restricted. To use in food packaging, the film thickness ranges from 10 to 60 xcexcm, preferably from 15 to 50 xcexcm. A film having a thickness less than 10 xcexcm has only an insufficient strength and therefore is liable to be broken when a food is wrapped therein. On the other hand, a film having a thickness exceeding 60 xcexcm shows an excessively high rigidity and frequently causes sealing failure. To use in envelope window film application, it is preferable that the film thickness is from 15 xcexcm to 50 xcexcm. A film having a thickness less than 15 xcexcm sometimes suffers from wrinkling in the course of window film application due to the small thickness and low rigidity. When the film thickness exceeds 50 xcexcm, there sometimes arises positioning error between the film and the envelope in the course of the window film application because of the relatively high film rigidity. It is still preferable that the film thickness ranges from 20 xcexcm to 40 xcexcm.
One face of the base film, which has been stretched into a definite thickness as described above, is subjected to a hydrophilic treatment.
The hydrophilic treatment may be carried out by using a publicly known method such as the chemical method with the use of conc. sulfuric acid or conc. nitric acid or the corona discharge method. It is suitable in the invention to use the corona discharge whereby both surfaces can be continuously treated one by one at a high speed. In the hydrophilic treatment, the concentration of conc. sulfuric acid or conc. nitric acid or the output of corona discharge is controlled so as to give the definite ratio of the surface tension of the film base surfaces (A) to the surface tension of the other film base surface (B). In case of using the chemical method, surface tension of 480 xcexcN/cm or 610 xcexcN/cm can be obtained by, for example, immersing the base film in nitric acid (purity: 96%) heated to 30xc2x0 C. respectively for 10 seconds or 60 seconds. In case of using the corona discharge method, the desired surface tensions can be obtained by using a four-crest electrode, adjusting the distance between the base film and the electrode to 1 mm and then subjecting the film base surface (A) and the film base surface (B) to the corona discharge respectively at 4 W/m2/min and 2 W/m2/min.
Next, the modifier composition dissolved in a solvent (for example, water, isopropyl alcohol) is applied by a publicly known method by using, for example, a roll coater, a spray coater or an air knife coater. After drying the solvent, the opposite surface, which has been coated with the modifier composition, is subjected to the hydrophilic treatment and then the film is wound into a roll. Alternatively, both surfaces of the film base may be preliminarily subjected to the hydrophilic treatment. It is favorable that the solvent is dried with a hot air stream of 70xc2x0 C. to 140xc2x0 C.
In the step of winding into a roll, the winding tension is preferably adjusted to 2 kgxc2x7m to 10 kgxc2x7m as in common cases. By winding into a roll, the film base surface (A) comes into contact with the film base surface (B) and thus the modifier composition applied on the film base surface (A) transfers onto the film base surface (B) (i.e., back transfer). When the film roll is unwound and used, the coated film surfaces (Axe2x80x2) and (Bxe2x80x2) can be thus presented. The face pressure to be applied to the roll preferably ranges from 0.05 to 100 kg/cm2. After storing at ordinary temperature for 4 hours or longer, the modifier composition can be transferred onto the base surface (B) at a desired ratio before using.
So long as the face pressure is maintained within the range as defined above, any troubles (for example, buckling) scarcely arise in any roll shape or winding tension ensuring the uniform transfer.
More particularly speaking, the film coated with the modifier composition is wound into a roll and stored for a definite period of time. Then, it is slit into pieces having appropriate width and length depending on the purpose followed by reverse rolling. Then the thus reverse-rolled film is stored for a definite period of time before using. That is to say, transfer is performed onto the innermost layer of the coated film roll under appropriate winding pressure. In the subsequent step of reverse rolling, the outermost layer in the previous step serves as the innermost layer and thus transfer is performed onto this face similarly.